The present application relates to a storage element including a storage layer, in which a magnetization state of a magnetic material is stored as information, and a fixed magnetization layer, a magnetization direction of which is fixed, where the magnetization direction of the storage layer can be changed by applying a current. The present application also relates to a memory including such storage element, and can be favorably applied to a nonvolatile memory.
Apparatuses that carry out information communication, in particular, small-scale user appliances such as personal digital assistants have been widely used and the request for improved performance, such as higher integration, faster operation, and reduced power consumption has been made for elements such as memory and logic circuits constituting such appliances.
In particular, nonvolatile memory may be an essential component for improving the performance of appliances.
In the field of nonvolatile memory, various technologies such as semiconductor flash memory and FeRAM (Ferroelectric Random Access Memory) have already been commercialized, with research and development also being conducted to achieve even higher performance.
As a new type of nonvolatile memory that uses magnetic material, much attention has been drawn to the field of MRAM (Magnetic Random Access Memory) that uses a tunnel magnetoresistive effect. Such technology has advanced rapidly in recent years (see, for example, J. Nahas et al., IEEE/ISSCC 2004 Visulas Supplement, p.22).
MRAM includes small storage elements regularly arranged that record information and wiring, such as word lines and bit lines, so that each element can be accessed.
The respective magnetic memory elements each include a storage layer in which information is stored as the magnetization direction of a ferromagnetic material.
Each magnetic memory element uses a magnetic tunnel junction (MTJ) including the storage layer mentioned above, a tunnel insulating film (a nonmagnetic spacer film), and a fixed magnetization layer the magnetization direction of which is fixed. The magnetization direction of the fixed magnetization layer can be fixed by providing an antiferromagnetic layer, for example.
With this construction, since the “tunnel magnetoresistive effect” with which the resistance to a tunnel current flowing in the tunnel insulating film changes based on the angle between the magnetization direction of the storage layer and the magnetization direction of the fixed magnetization layer is generated, information can be written (recorded) using this tunnel magnetoresistive effect. The magnitude of such resistance is maximized when the magnetization direction of the storage layer and the magnetization direction of the fixed magnetization layer are antiparallel and is minimized when the directions are parallel.
For a magnetic memory element with this construction, information can be recorded (written) into the magnetic memory element by controlling the magnetization direction of the storage layer in the magnetic memory element using a combined current magnetic field generated by applying currents to both a word line and a bit line. In a typical construction, different magnetization directions (or “magnetization states”) are stored corresponding to “0” information and “1” information.
Japanese Unexamined Patent Application Publication No. H10-116490, for example, discloses a method of recording (writing) information into a storage element using asteroid characteristics and US Patent Application Publication No. 2003/0072174, for example, discloses a method of recording (writing) information into a storage element using switching characteristics.
On the other hand, recorded information can be read by selecting a memory cell via an element such as a transistor and using the tunnel magnetoresistive effect of the magnetic memory element to detect any difference in the magnetization direction of the storage layer as a voltage signal, thereby detecting the recorded information.
Compared to other nonvolatile memory, the main characteristic of MRAM is that since “0” information and “1” information are rewritten by reversing the magnetization direction of the storage layer formed of ferromagnetic material, information can be rewritten at high speed and almost infinitely (i.e., over 1015 times).
However, since it may be necessary to generate a comparatively large current magnetic field to rewrite recorded information in an MRAM, it may require to supply a large current (for example, from several mA to several tens of mA) to the address wires. This results in higher power consumption.
Also, since both write address wires and read address wires may be required in an MRAM, it has been difficult to miniaturize the structure of memory cells. Also, when elements are miniaturized, the address wires become finer, which makes it difficult to supply a sufficient current, and since the required current magnetic field increases due to the increase in coercivity, there is increased power consumption.
Accordingly, the elements in an MRAM have been difficult to miniaturize.
Research has been carried out into constructions that record information without using a current magnetic field. Among such constructions, U.S. Pat. No. 5,695,864, for example, discloses memories that use magnetization reversal by spin transfer so that the magnetization can be reversed using a smaller current.
Japanese Unexamined Patent Application Publication No. 2003-17782 discloses magnetization reversal by spin transfer where spin-polarized electrons that have passed through a magnetic material are injected into another magnetic material, thereby causing magnetization reversal in this other magnetic material.
That is, when spin-polarized electrons that have passed a magnetic layer a magnetization direction of which is fixed (i.e., a “fixed magnetization layer”) are injected into another magnetic layer a magnetization direction of which is not fixed (i.e., a “free magnetization layer”), torque is applied to the magnetization of the free magnetization layer. Once a current of a given threshold or greater is applied, the magnetization direction of the magnetic layer (i.e., the free magnetization layer) can be reversed.
For example, by applying a current in a direction that is perpendicular to the film surface, of a giant magnetoresistive element (GMR element) and/or a magnetic tunnel junction element (MTJ element) with a free magnetization layer and a fixed magnetization layer, it will be possible to reverse the magnetization direction of at least some of the elements.
In this way, by forming a storage element with a fixed magnetization layer and a free magnetization layer (storage layer) and changing the polarity of a current flowing to the storage element, the magnetization direction of the storage layer is reversed, thereby switching (rewriting) between “0” information and “1” information.
Recorded information can be read using the tunnel magnetoresistive effect in the same way as with an MRAM by providing a tunnel insulating layer between the fixed magnetization layer and the free magnetization layer (storage layer).
Magnetization reversal by spin transfer has an advantage in that even when the elements are miniaturized, it is possible to achieve magnetization reversal without increasing the current.
The absolute value of the current supplied to a storage element to achieve magnetization reversal is 1 mA or below for a storage element of a scale of around 0.1 μm, for example, and also falls in proportion to the volume of the storage element, which is advantageous for scaling.
In addition, since word lines for recording that were required in an MRAM may be unnecessary, there is a further advantage that the construction of memory cells is simplified.
Hereinafter, a storage element that uses spin transfer is referred to as an “SpRAM” (Spin transfer Random Access Memory) and a spin polarizing electron flow that causes spin transfer is referred to as a “spin injection current”.
There are great expectations on SpRAM as nonvolatile memory that is capable of lower power consumption and increased capacity while maintaining MRAM's advantages of high speed and of being rewritable an almost infinite number of times.
FIG. 1 shows a schematic cross-sectional view of a memory cell of a memory (SpRAM) that uses spin transfer according to related art.
To read information recorded in a memory cell, a diode, a MOS transistor, or the like is used to electrically select the memory cell. The memory cell shown in FIG. 1 uses a MOS transistor.
First, the construction of a storage element 101 that forms a memory cell of an SpRAM will be described.
A ferromagnetic layer 112 and a ferromagnetic layer 114 are antiferromagnetically coupled by disposing a nonmagnetic layer 113 in between. In addition, the lower ferromagnetic layer 112 is disposed in contact with an antiferromagnetic layer 111, and due to exchange interaction that acts between such layers, has strong unidirectional magnetic anisotropy. A fixed magnetization layer 102 is formed of these four layers 111, 112, 113, and 114. That is, the fixed magnetization layer 102 includes two ferromagnetic layers 112, 114.
A ferromagnetic layer 116 is formed so that the magnetization direction M1 thereof is comparatively easy to rotate. This ferromagnetic layer 116 forms a storage layer (free magnetization layer) 103. A tunnel insulating layer 115 is formed between the ferromagnetic layer 114 of the fixed magnetization layer 102 and the ferromagnetic layer 116, that is, between the fixed magnetization layer 102 and the storage layer (free magnetization layer) 103. The tunnel insulating layer 115 breaks the magnetic coupling between the magnetic layers 116 and 114 thereabove and therebelow and also transmits a tunnel current. In this way, a TMR (tunnel magnetoresistive effect) element is formed including the fixed magnetization layer 102 that is a magnetic layer with a fixed magnetization direction, the tunnel insulating layer 115, and the storage layer (free magnetization layer) 103 a magnetization direction of which can be changed.
In addition, the layers 111 to 116 described above, a base film 110, and a top coat layer 117 constitute a storage element 101 including a TMR element.
A selecting MOS transistor 121 is formed in a silicon substrate 120 and a connection plug 107 is formed on one diffused layer 123 of the selecting MOS transistor 121. The base film 110 of the storage element 101 is connected to this connection plug 107. Although not shown, the other diffused layer 122 of the selecting MOS transistor 121 is connected to a sense line via a connection plug. The gate 106 of the selecting MOS transistor is connected to a selection signal line.
The top coat layer 117 of the storage element 101 is connected to a bit line (BL) 105 disposed thereupon.
In a normal state, due to the strong antiferromagnetic coupling via the nonmagnetic layer 113, the magnetization M11 of the ferromagnetic layer 112 and the magnetization M12 of the ferromagnetic layer 114 are almost completely antiparallel.
Since the ferromagnetic layer 112 and the ferromagnetic layer 114 are constructed so that the product of film thickness and saturation magnetization is equal, the leaked component of the pole magnetic field is so small as to be negligible.
The resistance of the TMR element including the layers 114, 115, 116 changes based on whether the direction of magnetization M1 of the ferromagnetic layer 116 of the storage layer 103 and the direction of magnetization M12 of the ferromagnetic layer 114 of the fixed magnetization layer 102 on both sides of the tunnel insulating layer 115 are parallel or antiparallel. When the magnetizations M1, M12 are parallel, the resistance decreases, while when the magnetizations M1, M12 are antiparallel, the resistance increases. If the resistance of the TMR element (the layers 114, 115, 116) changes, the resistance of the entire storage element 101 will also change. By using such change, it is possible to both record information and to read recorded information. That is, by assigning the low resistance state to “0” information and the high resistance state to “1” information, for example, it is possible to record (one bit of) binary information.
Note that since the ferromagnetic layer 114 that is closest to the storage layer 103 out of the layers in the fixed magnetization layer 102 serves as a reference for the direction of the magnetization M1 of the storage layer 103 during the reading of recorded information, this ferromagnetic layer 114 is also referred to as the “reference layer”. To rewrite information of a memory cell or to read the information that has been recorded in a memory cell, it is necessary to supply a spin injection current Iz. This spin injection current Iz passes through the storage element 101, the diffused layer 123, and the bit line 105.
By changing the polarity of the spin injection current Iz, it is possible to change the spin injection current Iz that flows through the storage element 101 from upward to downward or from downward to upward. By doing so, it is possible to change the direction of the magnetization M1 of the storage layer 103 of the storage element 101, thereby rewriting the information in the memory cell.
Japanese Unexamined Patent Application Publication No. 2005-277147 discloses an SpRAM where the direction of magnetization of the storage layer of the storage element is reversed by applying a spin injection current to the storage element and also applying a bias current magnetic field to the storage element.
More specifically, for the construction shown in FIG. 1 for example, the spin injection current Iz is applied to the storage element 101 via the bit line 105 and a bias current magnetic field Hx (not shown) produced by the current (which is equal to the spin injection current Iz) flowing through the bit line 105 is also applied to the storage layer 103 of the storage element 101. By doing so, the direction of the magnetization M1 of the storage layer 103 can be efficiently changed.
Hereinafter, a diagram in which the state of a memory cell is expressed with the spin injection current Iz on the vertical axis and the bias current magnetic field Hx on the horizontal axis is referred to as a “phase diagram”. Note that in the case where the spin injection current Iz and the bias current that produces the bias current magnetic field Hx are pulse currents, a phase diagram is produced using the crest value of the pulse currents.
For the storage element 101 shown in FIG. 1, the magnitude of the spin torque that acts upon the magnetization Mfree (=M1) of the storage layer (free magnetization layer) 103 is proportional to the vector triple product Mfree×Mfree×Mref. Here, Mref is the magnetization (=M12) of the reference layer (ferromagnetic layer) 114.
Since the magnetization Mfree of the storage layer (free magnetization layer) 103 and the Mref of the reference layer (ferromagnetic layer) 114 are antiparallel in the initial state, the spin torque that acts at first is extremely small. Since the spin torque is small, the magnetization reversal current increases.
A typical phase diagram includes a hysteresis region, a region (a “0” state region) where the memory cell is in a low-resistance state, that is, a “0” state regardless of the initial magnetization state, a region (a “1” state region) where the memory cell is in a high-resistance state, that is, a “1” state regardless of the initial magnetization state, and an unstable operation region where the three regions mentioned above are mixed.
For SpRAM to function as a memory with a sufficient operating margin for actual use, the three regions (the hysteresis region, the “0” state region, and the “1” state region need to be separated sufficiently widely. The hysteresis region can also be referred to as a “bistable operation region”. The “0” state region and “1” state region can also be referred to as “monostable operation regions”.
FIG. 2 shows one example of a phase diagram measured for the storage element 101 shown in FIG. 1. FIG. 2 shows the case where the pulse width of the current pulses of the spin injection current Iz is set at 1 ns (nanosecond). The phase diagram shown in FIG. 2 is a state diagram showing the state of the memory cell with the pulse crest value of the spin injection current Iz on the vertical axis and the pulse crest value of the bias current magnetic field Hx on the horizontal axis.
By separating the bistable operation region (the hysteresis region 80) and the monostable operation regions (the “0” state region 81 and the “1” state region 82) in this phase diagram, stable operation becomes possible. As shown in FIG. 2, unstable operation regions 83 where the three states 80, 81, 82 are mixed appear at the upper right (the first quadrant) and the lower left (the third quadrant) corners. When such unstable operation regions 83 exist, the spin injection current Iz and the bias current magnetic field Hx used during a magnetization reversal operation are set so that the unstable operation regions 83 are avoided.
However, since the bistable operation region 80 appears over a wide area in the phase diagram shown in FIG. 2, unless the spin injection current Iz and the bias current magnetic field Hx are increased, the monostable operation regions 81, 82 are not reached. This means that as described earlier, it may be still necessary to increase the magnetization reversal current.